Freshman Satto Tonegawa’s cause of death was ruled self-inflicted asphyxiation, according to his death certificate filed with the Cambridge City Clerk’s office. Tonegawa, son of MIT Nobel laureate Susumu Tonegawa, was found dead in his MacGregor dormitory room last Tuesday. He was 18.

Tonegawa’s death is the second of the term. On Sept. 4, Nicolas E. Del Castillo, a sophomore, was found dead in his East Campus dormitory room. His death was also ruled a suicide.

“As a longtime member of the MIT faculty and as a parent myself, I am also keenly aware that the loss of Nicolas Del Castillo and Satto Tonegawa has touched our entire community, including those who may not have known them directly,” Grimson wrote. “I want to extend my own personal wish that each of you takes a bit of time away from your academic routine to reflect on your connections to our community and on your sense of personal well-being.”

(See sidebar for the full text of Chancellor Grimson’s email.)

Tonegawa was born in Boston and lived in Chestnut Hill, about six miles from MIT.